The invention relates to a hydraulic control arrangement, by means of which preferably a plurality of hydraulic consumers are supplied with pressure medium (hydraulic fluid) in a demand-feed-regulated manner. Such a hydraulic control arrangement is used, in particular, in mobile working machines, such as, for example, dredger loaders.
A hydraulic control arrangement with demand-feed regulation or on the load-sensing principle is known from EP 0 566 449 B1. In this known control arrangement, a variable-displacement pump is set as a function of the highest load pressure of the actuated hydraulic consumers, in each case in such a way that the pump pressure is above the highest load pressure by the amount of a specific pressure difference. The pressure medium flows to the hydraulic consumers via adjustable metering diaphragms which are arranged between an inflow line, into which the pressure medium is conveyed by the variable-displacement pump, and the hydraulic consumers. What is achieved by means of pressure compensators located downstream of the metering diaphragms is that, when a sufficient quantity of pressure medium is delivered by the variable-displacement pump, there is a specific pressure difference across the metering diaphragms, irrespective of the load pressures of the hydraulic consumers, so that the pressure medium quantity flowing to a hydraulic consumer then depends only on the opening cross section of the respective metering diaphragm. When the metering diaphragm is opened further, a greater pressure medium quantity must flow across it in order to generate the specific pressure difference. The variable-displacement pump is in each case adjusted in such a way that it delivers the required pressure medium quantity. This is also therefore also referred to as demand-feed regulation.
The pressure compensators located downstream of the metering diaphragms are acted upon in the opening direction by the pressure downstream of the respective metering diaphragm and in the closing direction by a control pressure which prevails in a rear control space and which normally corresponds to the highest load pressure of all the hydraulic consumers supplied by the same hydraulic pump. When, during a simultaneous actuation of a plurality of hydraulic consumers, the metering diaphragms are opened to an extent such that the pressure medium quantity delivered by the hydraulic pump adjusted to the limit is smaller than the total pressure medium quantity required, the pressure medium quantities flowing to the individual hydraulic consumers are reduced in the same proportion, regardless of the respective load pressure of the hydraulic consumers. This is therefore referred to as control with load-independent throughflow distribution (LUDV control). Hydraulic consumers activated in this way are known, in brief, as LUDV consumers. Since, in LUDV control, the highest load pressure is also sensed and, by a variation in the pressure medium quantity required, an inflow pressure which is above the highest load pressure by the amount of a specific pressure difference is generated by the hydraulic pump, LUDV control is a special case of load-sensing control (LS control).
For a plurality of hydraulic consumers, to which pressure medium flows in each case via a metering diaphragm with an upstream pressure compensator, which is acted upon in the closing direction only by the pressure upstream of the metering diaphragm and in the opening direction only by the load pressure of the respective hydraulic consumer and by a compression spring, there is no load-independent throughflow distribution. There is a straightforward LS control and LS consumer. Such a control is known, for example, from DE 197 14 141 A1. Here, when a plurality of hydraulic consumers are actuated simultaneously and there is not a sufficient pressure medium quantity delivered by the variable-displacement pump, only the pressure medium quantity flowing to the hydraulic consumer having the highest load pressure is reduced.
DE 197 03 997 A1 discloses a hydraulic control arrangement which includes a variable-displacement pump supplying pressure medium to a plurality of LUDV consumers and to one LS consumer. The LS consumer is a hydraulic steering system in which pressure medium preferentially has to be supplied. This priority of the hydraulic steering system in relation to the LUDV consumers is achieved by means of a priority valve which has a first junction, connected to a portion of the inflow line upstream of the metering diaphragm assigned to the steering system, and a second junction, connected to the load-signaling line, and a valve member of which is acted upon in the opening direction of the connection between the first junction and the second junction by the load pressure of the steering system and the force of a compression spring and in the closing direction of the connection between the first junction and the second junction by the pressure prevailing in the first junction. When the load pressure of the steering system is higher than the load pressure of an LUDV consumer actuated in parallel with this and a variable-displacement pump conveys sufficient pressure medium for all the actuated hydraulic consumers (saturation situation), the priority valve causes so much pressure medium to flow out of the inflow line into the load-signaling line that a pressure dependent on the height of the load pressure of the steering system prevails in the latter and a pressure which is above the steering system pressure by the amount of a specific pressure difference prevails in the inflow line. When so much pressure medium is demanded that the variable-displacement pump is adjusted as far as the maximum stroke volume (undersaturation situation), then the priority valve increases the pressure in the load-signaling line to an extent such that, by a more pronounced throttling of the pressure medium stream, the LUDV pressure compensators lower the pressure difference across the LUDV metering diaphragms to an extent such that the pressure in the inflow line maintains the same level as in the saturation situation and the pressure difference across the LS metering diaphragm is exactly the same as in the saturation situation.
A control arrangement with demand-feed regulation can be implemented not only with a variable-displacement pump having an LS pump-regulating valve, but also with a fixed-displacement pump and an LS-controlled bypass pressure compensator, via which pressure medium conveyed by the hydraulic pump and not required by the hydraulic consumers flows back to a tank. In this case, however, the losses of nonutilizable energy are higher than when the variable-displacement pump is used.
A hydraulic control arrangement is known from DE 34 22 205 A1, FIG. 4. In this, the two hydraulic pumps are fixed-displacement pumps and the two demand-feed regulators are bypass pressure compensators. An inflow line leads from the pressure outlet of the second hydraulic pump to a control block, while the first hydraulic pump conveys into the inflow line via a nonreturn valve. The two pressure compensators are acted upon in the closing direction by the pressure in the load-signaling line and a compression spring and in the opening direction by the pressure in the inflow line. By causing the compression spring to exert on the second pressure compensator assigned to the second hydraulic pump a force which corresponds to a somewhat greater pressure difference than the compression spring on the first pressure compensator assigned to the first hydraulic pump, in the case of a low demand for pressure medium, only the second hydraulic pump conveys pressure medium to the control block, while the pressure medium quantity conveyed by the first hydraulic pump flows back, pressureless, to the tank via the first pressure compensator. When the demand for pressure medium of the simultaneously actuated hydraulic consumers exceeds the conveyed quantity of the second hydraulic pump, the pressure in the inflow line falls below a value determined by the highest load pressure and by the pressure difference set at the second pressure compensator. The second pressure compensator closes completely. The first pressure compensator assumes a regulating position and ensures that the pressure in the inflow line is above the highest load pressure by the amount of the pressure difference set at the first pressure compensator. In the known control arrangement, the two hydraulic pumps and the two bypass pressure compensators could also be readily replaced by two variable-displacement pumps with LS pump-regulating valves set to different pressure differences.
Irrespective of whether fixed-displacement pumps or variable-displacement pumps are used, the known hydraulic control arrangement has a disadvantage in as much as, after an appropriate setting of the pressure compensators or of the pump-regulating valves, it is fixed as to which of the two hydraulic pumps first alone supplies pressure medium to the hydraulic consumers and which is connected when the demand for pressure medium exceeds the (maximum) pressure medium quantity conveyed by the second hydraulic pump. This is a disadvantage, in particular, when the hydraulic pumps are of different size in respect of the conveyed quantity or when a further hydraulic consumer, for example a steering system, can be supplied with pressure medium solely by one hydraulic pump.
The object on which the invention is based is, therefore, to develop further a hydraulic control arrangement with two hydraulic pumps and with other features mentioned-above, in such a way that a more flexible selection of the hydraulic pump conveying under pressure, in terms of the demand for pressure medium of the hydraulic consumers, is possible in a simple way.
The intended object is achieved, according to the invention, in that a nonreturn valve opening toward the inflow line is also arranged between the second hydraulic pump and said inflow line, and in that there are means by which the action of force upon the demand-feed regulator can be changed in such a way that the hydraulic consumers can be supplied with pressure medium both solely by the first hydraulic pump and solely by the second hydraulic pump. According to the invention, therefore, the desired flexibility is acquired in a very simple way by a change in the action of force upon the regulating pistons of the demand-feed regulators.
Thus, according to other features of the invention, a first advantageous possibility for influencing the demand-feed regulator is for the spring on at least one demand-feed regulator to be adjustable in such a way that the pressure difference corresponding to the force of the springs is, on the one hand, greater and, on the other hand, smaller on the first demand-feed regulator than on the second demand-feed regulator. Preferably, in this case, therefore, the pressure difference determined by the spring on the one demand-feed regulator in a first state is smaller than the pressure difference on the other demand-feed regulator. The spring, then, can be prestressed to a greater extent and thereby brought into a second state, in which the pressure difference determined by it is greater than the pressure difference on the other demand-feed regulator. It is also conceivable, however, to prestress the two springs approximately to the same extent in a state of rest and to stress one spring or the other to a greater extent in order to supply pressure medium to the hydraulic consumers by means of only one or the other hydraulic pump.
According to still other features of the invention, a second advantageous possibility for influencing the demand-feed regulator is that a control space on the second active surface of the second demand-feed regulator is connected to the inflow line, that the load-signaling line can be shut off relative to the first demand-feed regulator by means of a switching valve, that, in the blocking position of the switching valve, the first active surface of the first demand-feed regulator can be relieved of pressure, and that the spring of the first demand-feed regulator is set at a higher pressure difference than the spring of the second demand-feed regulator. A slightly different setting of the demand-feed regulators and only one switching valve is necessary here.
According to yet other features of the invention, a third advantageous possibility for influencing the demand-feed regulators is that the load-signaling line can be shut off relative to the first demand-feed regulator by means of a first switching valve and relative to the second demand-feed regulator by means of a second switching valve, and that, in the blocking position of one switching valve, the first active surface of the corresponding demand-feed regulator can be relieved of pressure. In this case, as compared with conventional control arrangements, there is no need for any changes to the demand-feed regulators. Moreover, the two demand-feed regulators can be set at the same pressure differences between the pump pressure and the highest load pressure, so that, in situations where only one hydraulic pump or only the other hydraulic pump supplies pressure medium to the hydraulic consumers, there is the same pressure difference across the metering diaphragms.
If a control space on the second active surface of a demand-feed regulator is connected, upstream of the corresponding nonreturn valve, to a flow path running between the corresponding hydraulic pump and the nonreturn valve, then the control space and a working junction of a demand-feed regulator can be connected to one another in a simple way. However, in this case, when one hydraulic pump is not being used, the demand-feed regulator sets a low-level pump pressure, so that, even in the case of a variable-displacement pump, some losses of nonutilizable energy still occur. It seems more favorable, therefore, in terms of the energy balance, if, as specified in patent claim 6, a control space on the second active surface of a demand-feed regulator is connected to the inflow line. This is because the demand-feed regulator is then held, by a pressure which prevails in the inflow line and is higher than the pressure equivalent to the force of the spring, in an open end position in which the pressure medium quantity conveyed by a fixed-displacement pump can flow out, pressureless, to the tank or a variable-displacement pump is adjusted completely in the direction of zero conveyed quantity.
According to another feature of the invention, the two hydraulic pumps are of different size in terms of the stroke volume, so that good adaptation to the operating state and to the pressure medium demand dependent on this is possible.
According to yet another feature of the invention, there may be a further hydraulic consumer, to which pressure medium can be supplied only by the one hydraulic pump. If this further hydraulic consumer is an LS consumer which is to have priority in the supply of pressure medium, for example a hydraulic steering system, then, if the other hydraulic consumers are LUDV consumers, priority may be ensured by a prestressing of the LUDV pressure compensators of the other consumers which exceeds the amount necessary per se. In particular, in the design according to further features of the invention, the activation of the preferred consumer is not influenced by the other consumers. However, the prestressed closing springs of the LUDV pressure compensators cause energy losses. Even though a relatively high outlay in terms of apparatus is involved, a design according to additional features of the invention therefore seems more favorable in terms of the energy balance, whereby, in an undersaturation situation, the pressure on the rear side of the LUDV pressure compensators is increased beyond the highest load pressure and consequently the pressure difference and the pressure medium flow across the second metering diaphragms are reduced.
The hydraulic pumps may be those with a constant stroke volume, to which a bypass pressure compensator is assigned, or else variable-displacement pumps.